Method of producing ribbed-metal sandwich structures



R. I. JAFFEE June 3, 1969 METHOD OF PRODUCING RIBBED-METAL SANDWICHSTRUCTURES Filed June 21, 1967 Sheet T I w Z m VK YA MW/6 B/ 2 M m\l R.I. JAFFEE June 3, 1969 METHOD OF PRODUCING RIBBED-METAL SANDWICHSTRUCTURES z of 2 Sheet Filed June 21. 1967 F/G. 4A

United States Patent U.S. Cl. 29-423 9 Claims ABSTRACT OF THE DISCLOSUREA method for partially removing metal filler members from roll-bonded,ribbed-metal structures, which consists of providing bimetallic fillermembers formed with cores of a metal disposed for independent tensileelongation relative to their surrounding cases and separating thesecores from their cases after roll bonding by exerting a tensile force onsaid cores so as to efiect independent tensile elongation relative tothe cases to eifect a reduction in cross-sectional dimensions of thecores and cause them to separate from the fillers. The cores of saidfillers may then be removed by mechanical means. The crosssectionaldimensions of the cores are preferably shaped with a high ratio ofheight-to-width so that after being elongated in a direction parallel tothe direction of rolling, the ratio of height-to-width of the cores isapproximately unity.

BACKGROUND This invention relates to improvements in the manufacture ofroll-bonded hollow-metal ribbed structures for structural use such as inthe aerospace, marine, and transportation industries and relates inparticular to the use of fillers that have mechanically removable coreswhich may be removed after roll bonding so as to remove at least a partof the filler and facilitate removal of the remainder by chemicalleaching.

Lightweight panels, consisting of spaced metal sheets separated by metalribs, of exceptional strength may be manufactured by roll-bondingtechniques such as is described in U.S. Patent 3,044,160. Particularlydesirable structures of this type are described in my copending patentapplication Ser. No. 410,971 filed Nov. 13, 1964, and entitled SandwichStructures and Method. Stiffened skin assemblies having angularlypositioned ribs (juxtapositioned V-shaped structures) such as depictedby FIGS. 11 to 15 of that patent application are particularlysignificant when utilized in the construction of high-speed aircraft orspacecraft vehicles.

The prior known technique employed for removing the matrix metal orspacers positioned between the ribs for support during roll bonding hasbeen to corrode away these members by flowing a chemical reagent ontothe panel ends. Such a procedure is slow and inefiicient. Additionally,long time exposure of the titanium to the effects of such corrosivechemicals sometimes results in corrosion or erosion of portions of theribbed structure.

It is obvious that if a post-fabrication throughhole in made in eachfiller bar, the leaching rate would be greatly accelerated. Furthermore,the volume of material to be leached is significantly reduced. Providingsuch passageways by mechanical techniques, such as drilling, is notpractical, since this would be a time-consuming, laborious, andexpensive task and would be confined to small flat panels whereexcessively long drill bits would not be required.

My technique for providing passageways through the filler bars is toutilize bimetallic filler bars that have a metal core capable of beingphysically removed from the 3,447,231 Patented June 3, 1969 assemblyafter roll bonding is completed. This may be accomplished by applyingthe bimetallic bar structures utilized for hollow-drill steelmanufacture to the rollwelded sandwich process. This structure has acore that dilfers in its mechanical properties from its case in a mannerthat the core can be separated and removed from the case. This isaccomplished by providing a core metal that possesses greater toughnessthan the case and which is disposed to uniformly deform when placed intension. For example, one such material is austenitic steel stabilizedwith manganese and/or nickel. Such composite structures may be shaped toform the spacers or fillers utilized in the sandwich structureroll-bonding process. At the conclusion of roll bonding, the ends of theaustenitic steel cores are exposed and placed under suflicient tensionto cause them to elongate. The toughness and elongation properties ofsuch a core causes them to separate from the mild steel core so thatthey can be readily removed to provide the desired passageways forsubsequent leaching.

Due to the fact that the core material in the bimetallic fillerassemblies possess greater toughness than the case metal, such coresoffer greater resistance to deformation during roll bonding of theassembled hollow rib structure pack. As a result where such cores are inthe form of rounds, they cause uneven lateral pressure to be exerted onthe ribs of the structure which effects some rib dis tortion ordeformation (bending) during rolling. Such rib distortion materiallydetracts from the strength properties of the ultimate product.

I have found that rib distortion occasioned by using spacers having arelatively tough core can be reduced or eliminated by providing a spacerwith a core with a high height-to-width ratio that is preferably shapedsimilar to the spacer itself, e.q., isoceles triangular core in anisoceles triangular spacer, rectangular core within a rectangularspacer, etc.

DRAWINGS FIG. 1 is an enlarged cross-sectional view of a pack assemblyprior to roll bonding showing triangular steel filler bars machined fromconventional round-cored steel bars (used to manufacture the so-calledhollow-core drill rod).

FIG. 2 is an enlarged fragmented cross-sectional view of a portion ofthe ribbed structure of FIG. 1 after roll bonding showing the resultingoval shape of through holes after core removal, exhibiting somedistortion of the ribs in the section adjacent to the cores.

FIG. 3 is an enlarged fragmented cross-sectional View of a portion of aribbed structure similar to that of FIG. 1 before roll bondingconstructed in accordance with the preferred method of the presentinvention showing shaped, cored, filler bars.

FIG. 3A is an enlarged fragmented cross-sectional View of the structureof FIG. 3 after roll bonding.

FIG. 4 is an enlarged fragmented cross-sectional view of a ribbedstructure showing triangular steel bar spacers and complementary shapedcore.

FIG. 4A shows the structure of FIG. 4 after roll bonding.

FIG. 5 shows an enlarged fragmented cross-sectional view of a ribbedstructure having rectangular spacer bars and rectangular cores.

FIG. 5A shows the structure of FIG. 5 after roll bonding.

DESCRIPTION In the drawings, FIG. 1 is a cross-sectional view of a packconstructed in accordance with the teachings of my Patent No. 3,044,160and copending patent application (Ser. No. 410,971 filed Nov. 13, 1964).The titanium and abut these members. Ribs 14 are separated by thespacers 1-6 which are appropriately shaped to position the ribs in adesired V shape configuration.

Spacers 16 are constructed of drill core stock having a core portion 18which is made from an austenitic manganese steel that has greatertoughness than the surrounding mild steel.

The assembly 8 designed for roll bonding and leaching is positionedwithin a steel yoke 20 to which there is welded steel cover plates 22and 24 and end plates (not shown).

Air is withdrawn from the assembly and, in accordance with the teachingsof the aforementioned patent application and issued patent, the assemblyis roll bonded to obtain the structure of FIG. 2.

The structure of FIG. 2 consists of the ribbed metal structure afterroll bonding and removal of cover plates 22 and 24 and yoke 20 butbefore leaching of the spacer members 16. The manganese austenitic steelcores 18, however, have been removed by exposing their ends and applyingsufiicient tension stress to cause them to elongate and separate fromthe mild steel casings (due to a reduction in cross-sectionaldimensions). They were then simply pulled from the assembly to leavepassageways 26. The remainder of the separators may now be removed bysimply passing a corrosive acid through passageways 26 that willdissolve the mild steel but which will not attack the titanium basemetal.

It will be noted that the passageways 26 of the assembly of FIG. 2 werereduced in height. This, of course, is caused by hot rolling whichreduces the gage of the assembly, and, consequently, flattens thespacers 16 including the initially round austenitic steel cores 18.

Since the cores 18 are tougher than the mild steel of spacers 16 theyexert lateral pressure on the ribs 14 during roll bonding to causedistortion (see FIG. 2). Such rib distortion materially weakens theribbed-metal structures and detracts from their usefulness in the airand space vehicle industry.

I have found that the rib distortion experienced during roll bondingwhen utilizing hollow drill steel-type cores may be materially reducedor eliminated by providing spacers with cores that are cross-sectionallyshaped similar to the spacer itself prior to roll bonding. This type ofassembly is shown by FIG. 3 wherein a titanium rib structure 38positioned within a yoke and cover plates (not shown) is provided withspacers 46 having cores 48 formed with a high height-to-width ratio.

Cores 48 are positioned so that when the pack assembly is hot rolled toeffect roll bonding or welding of the ribs 34 to the skins 30 and 32 theelongated cores become round (see openings 50 of FIG. 3A). Since thecores 48 do not exert lateral pressure, ribs 34 are not distorted.

It will be appreciated that the degree of distortion of the ribs ofribbed-metal sandwich structures caused by the spacer or filler cores isrelative. The greater the reduction in gage of the roll pack the greaterthe degree of distortion. In a similar manner, the desired ratio ofheight over width of the cores (such as cores 48) will vary with thesize of the pack assembly, reduction in gage or thickness during rollbonding, etc. These parameters must be determined for each individualassembly, however, any high ratio (greater than 1:1) of height-to-widthwill have some advantageous effect in reducing rib distortion.

Further, the core need not be oval shaped in cross section. For example,it may be desirable to utilize a roughly elongated triangularconfiguration that follows the cross-sectional triangular configurationof a spacer similar to spacers 16 and 46 of the drawings or it may bedesirable to utilize a generally rectangular elongated 4 1 core wherethe ribs of the ribbed-metal sandwich structure are vertically disposed.

FIG. 4 shows a hollow-ribbed pack structure prior to roll bonding (yokeand cover plates, not shown) wherein the triangular shaped filler bars52 are provided with complementarily shaped austenitic cores 54. FIG. 4Ashows the same structure after roll bonding and gage reduction (thecores havingbeen removed). The core shape after rolling had the final(flattened) contour of passageways 56. Ribs 58 are not distorted.

In the embodiment of FIG. 5 hollow-ribbed structure assembly 60 isprovided with rectangular fillers or spacers 62 which are provided withhigh height-to-width rectangular cores 64. FIG. 5A shows the samestructure as FIG. 5 after roll bonding and core removal. Holes 66 showthe final contour of the cores after hot rolling and before leaching.

The tough core material may, of course, be fabricated by preshaping thecore and machining a complementary shaped hole to receive the shapedhole in the filler metal. Shaping may, of course, also be effected bycontrolled mechanical deformation of the bimetallic structure (i.e.,rolling).

An advantage of the method of the present invention relates to the greatsavings effected in leaching time. After providing the hollow coresleaching times are cut from days and weeks to less than an hour.

Another significant advantage of the present invention relates toforming. Ribbed structures must be formed (bending) with the filler barsin place to avoid rib buckling. I have found that after removing thecores but before leaching, the panels can be bent to a smaller radiusthan with solid fillers.

Although any through-hole effected in the filler bars contributessignificantly to the advantageous features discussed above inconjunction with the utilization of the method of the present invention,it is preferred that the cross sectional surface area of the tough coreconstitute from about 5 percent to 50 percent of the area of thecomposite filler bar. Optimum results have been experienced where thecore consists of about 15 percent of the cross-sectional area of thecomposite bar. If the core area is less than about 5 percent of thefiller bar, it becomes difficult to accomplish acid leaching and wheresuch area exceeds 50 percent there is danger of rib buckling (whenforming the completed structure after core removal).

The means for manufacturing core spacers such as sapcers 16, 46, 52, and62 are well known (particularly in the hollow-drill steel art). Abimetallic assembled unit is hot rolled to obtain the rod-core assembly.

The core metal or the case metal may consist of any metal as long as thecore metal is tougher than the case metal and is capable of relativelyuniform elongation and the case metal is capable of being leached oreroded from the ribbed metal structure. I have had particular success inthe manufacture of titanium ribbed metal structures in using a mildsteel case and a high manganese (austenitic) steel core. Austeniticsteels in general possess satisfactory uniform elongation properties andare tougher than mild steel grades. Particularly useful austeniticsteels for core materials (in mild steel) are those which form somemartensite (or bainite) in their structure upon the application ofstrain. These are the high manganese grades such as A1S1 Type 200, 201,etc. Other particularly useful grades are the precipitation hardeningaustenitic steels.

The tough core material may, of course, be elongated by simply rollingthe bimetallic assembly along one direction prior to forming thespacers.

Although the examples (below) and the description above are directed tothe manufacture of titanium ribbedmetal sandwich structures, it will beappreciated that the basic principles of the present invention areapplicable to many metals (i.e., Zr, Hf, stainless steel, etc.).

The following specific examples serve to illustrate the presentinvention and in no way limit the claims to the exact embodiment setforth:

Example I A small pack 18 x 8 inches was assembled using 12- inch-longcomposite filler bars. These bars were machined from l-inch-diameterAtlas Steel Corporation Ottawagrade hollow-drill steel stock. TheOttawa-grade drillsteel stock consists of a 1080 carbon steel outerjacket (approximately /2 inch base x inch high) and a tough manganeseaustenitic steel core A inch diameter) which is mechanically extractedafter fabrication. The filler bar design was the same as thatillustrated by filler bar 16 of FIG. 1.

The pack was assembled so as to allow the filler bars to extend beyondthe sandwich structure after rolling to provide a gripping area formechanical extraction. After rolling, the total sandwich structurelength was 17 /2 inches with 6% inches of filler bars extending beyondthe sandwich structure at each end.

The pack was rolled on 60-inch production plate mill using standardpractice developed for production rolling of Ti-6Al-4V alloy panels. Agas-fired horizontal batch furnace was used to heat the pack forrolling. After rolling, the carbon steel yoke material was trimmed fromthe sides and ends of the panel with an abrasive cut-off wheel. Thecarbon steel pack covers were then stripped mechanically from the panel.Transverse saw cuts were made about 1 inch beyond the sandwich structureends through the mild steep spacers and to a depth of about 1/ 16 inchin the 1080 carbon steel filler bar jackets. The entire assembly wasthen placed in a tensile machine and loaded in tension. The 1080 carbonsteel jackets fractured at the saw cut on one end of the panel and werestripped away exposing the austenitic steel cores. The specimen was thenreloaded in the tensile machine so that the bare austenitic steel coreswere gripped in the machine on one end of the panel and the carbon steeljackets gripped on the opposite end. The specimen was loaded in tensionand the austenitic steel cores were gang-pulled from their surroundingsteel jackets leaving a continuous hold through each filler bar.

The result was a panel that looked very much like the one illustrated bythe drawing of FIG. 2.

Example II Two-and-one-half inch lengths of 1018 steel filler wedgeswere drilled longitudinally, and inch lengths were cut from each end.Each composite bar was assembled over approximately machined Manganolrods so that the longer portion was sandwiched between the two separateinch lengths (for gripping). A lime wash was applied to the drilled holeof the longer center portion. This was to facilitate mechanical partingof components in these regions after fabrication.

A 2 inch long, 5 /2 cycle Ti-6A1-4V corrugation was mated to the centralportions of the filler bars. Spaces between the end (gripping) portionsof the core assembly were shimmed with steel strip to position theTi-6A1-4V corrugation over the center filler segments. Titanium- 6A1-4Vcovers overlapped the corrugation section by inch on each end. Thisassembly was encapsulated in the usual steel yoke and cover assembly,and routinely fabricated to 60 percent reduction (hot rolling).

After fabrication, yoke sides were cut away, and pack and sandwichcovers were ground away. The ends of this panel were gripped in atensile machine and pulled to failure. Two of the Manganol cores weresuccessfully removed.

I claim:

1. A method for providing passageways in the spacer members between theribs of a roll-bonded ribbed-metal sandwich structure, comprising:

(a) positioning bimetallic spacer members, formed with metal cores thatexhibit greater toughness than the case metal and which are disposed toelongate uniformly, between the ribs of a pack assembly disposed toprovide said roll-bonded ribbed-metal sandwich structure when hotrolled;

(b) hot rolling said pack assembly at a temperature and pressuredisposed to provide said roll-bonded ribbed-metal sandwich structure;

(0) applying tensile stress to said cores of a magnitude to cause saidcores to elongate and shrink in crosssectional dimensions so as to breakaway from said case metal; and

(d) removing said separated cores from said roll-bonded ribbed-metalsandwich structure.

2. The method of claim 1 wherein said cores are formed with a high ratioof height-to-width.

3. The method of claim 1 wherein said cores and case are constructed ofsteel and said ribbed-metal sandwich structure consists of titanium ortitanium alloy.

4. The method of claim 3 wherein said case is constructed of mild steeland said core consists of an austenitic steel.

5. The method of claim 4 wherein said core consists of a high manganesegrade of ausenitic steel.

6. The method of claim 1 wherein said cores have a complimentarycross-sectional shape to the cross-section shape of said case.

7. The method of claim 2 wherein said cores have a substantiallycomplimentary cross-sectional shape to the cross-sectional shape of saidcase.

8. The method of claim 1 wherein the relative size of said metal coresto the balance of said spacer members is such as to occupy from 5percent to 50 percent of the cross-sectional area of said members.

9. The method of claim 1 wherein the relative size of said metal coresto the balance of said spacer members is such as to occupy about 15percent of the cross-sectional area of said members.

References Cited UNITED STATES PATENTS 3,044,160 7/1962 Jatfee 29--4233,061,713 10/1962 Eggert. 3,321,826 5/1967 Lowy 29423 3,380,146 4/1967Babel et a1. 29423 JOHN F. CAMPBELL, Primary Examiner.

J. L. CLINE, Assistant Examiner.

US. Cl. X.R. 29497.5, 504

